Depends if you are expecting good neighbors or bad. If the latter, then maybe a blackberry patch with sickle-shaped thorns is in order. (ha,ha) Seriously, a lot of your prairie grasses put down deep roots, much deeper than you might expect. With the nutrients and moisture of the leach bed, root invasion into the leach bed is definitely possible. Where you live, your leach bed lines might be below the frost line at probably 3' to 4' down. Around where I live, I've seen trees in close proximity to leach beds for years with no collapse of the leach bed system. However, you will know if you have any roots building up in the system with an increase in the number of times that your septic tank will need pumped out. Prairie Cord grass is not that tall as to screen out the neighboring house. What is the purpose of the grass that you want?

Spartina pectinata (Prairie Cordgrass) rhizomes seldom exceed 2 feet deep. The mature plant is ~5' tall. It's an aggressive spreader.

You can buy seeds, rhizomes or plugs at the link below.

Here is a link that might be useful: Prairie Cordgrass

I guess the main purpose was to have something beautiful to look at--the view is right outside my home office window and I thought it would be pretty to have a septic garden over the leach field. My house is actually 3 stories in the back (walkout basement) so I know I would never have anything tall enough to give privacy in that spot but I didn't want it to be bare either.

I love grasses and thought it would be awesome to have something there. Along with some other non dangerous plants.

I live in a prairie setting and the lot behind me is vacantand has been for years but I'm sure someone will eventually build and when they do our houses will be somewhat close due to lot shape. So that's another thing to think about- (controlling the spread of the grass) but I'll worry about that down the road if/when it happens.

You've got a lot of options in grasses. Sweeps of various native prairie grasses, like Prairie Cordgrass, Prairie Dropseed and the Bluestems, dotted with clumps of Panicum, Miscanthus and other ornamentals would look great.